Integral microwave radiating and generating unit for heating



June 3, 1969 w, scoT 7 3,448,384

INTEGRAL MICROWAVE RADIATING AND GENERATING UNIT FOR HEATING Filed Oct. 23, 1965 Sheet of 6 IN VENTOR. 4114 M/ 5207f June 3, 1969 A. w. sco'r'r 3,448,384

INTEGRAL MICROWAVE RADIATING AND GENERATING UNIT FOR HEATING INVENTOR.

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June 3, 1969 A. w. s,co'r'r INTEGRAL MICROWAVE RADIATING AND GENERATING UNIT FOR HEATING Filed on. 23. 1965 Sheet [k INVENTOR.

June 3, 1969 A. w. SCOTT 3,448,384

INTEGRAL MICROWAVE RADIATING AND GENERATING UNIT FOR HEATING Filed Oct. 23, 1965 Sheet 4 of e INVENTOR. 4144/ M/ 620W INTEGRAL MICROWAVE RADIATINGAND GENERATING UNITFOR HEATING Filed QCt. 23, 1965 A- W. SCOTT June 3, 1969 Sheet 5" of s.

v r/vy. .4 rll. 5 g l N\\\\N N \N\'\\\\\\ V/V /V////- NQ R NNNNNNNNNNNNNNN \7/ /A// /A//A// A. W. SCOTT June 3, 1969 INTEGRAL MICROWAVE RADIATING AND GENERATING UNIT FOR HEATING Sheet Filed Oct. 23, 1965 United States Patent Ofice 3,448,384 Patented June 3, 1969 U.S. Cl. 325-120 23 Claims ABSTRACT OF THE DISCLOSURE A pair of dielectric sheets rorm a flat vacuum space in which a fiat electron beam is created. The essential operating structure for a microwave tube are printed on the dielectric sheets. For example, focusing of the electron beam is achieved through printed electrostatic focusing members. A printed microwave interaction structure and a printed microwave antenna are also provided. The various collector plates for the electron .beam may be printed or fabricated on metal structures so as to serve the dual function of beam collection and thermal radiation for food browning. A printed power control circuit which is sensitive to a sampling of microwave output power varies the beam location relative to the printed microwave interaction structure so as to assure a constant power for the microwave unit in spite of short or long-term variations in an ordinary line voltage supply source.

This invention relates in general to electronic heating apparatus utilizing ultrahigh frequency energy and particularly relates to microwave heating apparatus for providing uniform heating in a cavity, said apparatus comprising a compact and integral generating and radiating unit which forms substantially one side of the cavity to be heated.

Microwave generators for heating, commonly referred to as microwave oven units, are known in the art. The existing prior .art microwave oven uni-ts generally comprise a plurality of electronic components, each of which perform a single function, and the majority of which components are external of the cavity, or oven enclosure. These units of the prior art, depending upon alternating or direct current power requirements, normally include a complex rectifier, filter, and high voltage transformer and high voltage leads; such equipment being required to convert standard line voltage to the high valued voltage necessary for powering a standard magnetron tube. The magnetron tube is a primary component of prior art microwave oven unit-s, and is located externally of the cavity. The magnetron serves to convert high voltage electrical energy to microwave energy. In operation, the magnetron tube consumes a significant amount of energy and generates considerable heat. Accordingly, the prior art ovens require a fan for air circulation or a pump for liquid circulation to cool the magnetron tube. Such equipment is expensive, creates electrical disturbances, and increases the bulk of prior art microwave ovens to the extent that they are not satisfactory or practical for home or industrial use.

Normally connected to the magnetron tube is an external wave guide having dimensions precisely designed and machined so as to conduct the microwave energy generated by the magnetron tube to the oven cavity. This wave guide generally "terminates at a stub or point source located at a small opening in one wall of the cavity. Positioned near the opening is .a mechanically rotating or reciprocating stirrer for uniformly dispersing the radiated microwave energy throughout the cavitys enclosure. These stirrers of the prior art are not entirely satisfactory in dispersing microwave energy in the oven, with the result sometimes being the presence of hot and cold spots rather than an even distribution of heating energy. In addition, such mechanical parts represent additional expense, are unreliable and unattractive and thus greatly detract from the commercial acceptance of prior art microwave ovens.-

In summary, the prior art microwave ovens are complex, costly, and normally unsuitable for many microwave oven uses in that they are very bulky and require many external components and dangerous high voltages, making such units unattractive and impractical for commercial and home use. The foregoing disadvantages of the prior art are avoided in accordance with the principle of this invention wherein a new and improved microwave generating and radiating unit is provided as one integral component. It is compact and may be mounted internally as one side of the oven cavity. The microwave unit of this invention is a low-voltage unit that operates directly from conventional household line voltage, and thus requires no external magnetrons or external cooling equipment which characterize the prior art.

Printed circuitry on dielectric sheets is employed for substantially all operating structures for this microwave unit including a significant portion of the power conversion circuitry, the electron beam creating and collecting elements, the electron beam focusing means, microwave interaction structure, the microwave antenna or radiating means, and feed back means. A plurality of microwave units chosen with individual and different operating frequencies may be positioned side by side in a common vacuum envelope on one wall of the cavity to be heated. Each unit has its own printed circuit antenna. These antenna structures are of a random pattern. The microwave signals from the plurality of units are noncoherent, and these factors combine to radiate microwave energy uniformly throughout the cavity, thus eliminating all mechanical stirring apparatus. Printed circuit structure of this invention is readily adaptable for automation and, accordingly, the unit of this invention represents not only a decrease in size and voltage requirements, but also represents a decrease in the cost of manufacturing as compared to prior art microwave oven units.

'One further advantage of the structure of this invention may be appreciated by considering that prior art apparatus utilizes the foregoing described magnetron, external wave guide and mechanical stirrer to supply a deep cooking effect on food placed within the oven cavity. To be acceptable for commercial or home use, the high frequency unit must also incorporate means for browning the food. Such browning means in prior art microwave ovens has generally taken the form of convention-a'l high resistance oven coils. Elaborate procedures and design criteria are required in the prior art in order to design and locate this radiant heat coil in a manner and at a position so as to avoid wasteful interference and absorption of the microwave energy. Such conventional browning coils also increase the power requirements, cost and complexity of prior art units. In preferred embodiments, the microwave unit of this invention, avoids the above-mentioned disadvantages in that electrodes for collecting the electron beam are positioned internally of the unit and are shaped as thermal radiators. Accordingly the browning requirements for food in the oven are supplied by an integral part of the unit of this invention. These thermal radiating means thus serve a dual function of collecting the electron beam and simultaneously supplying thermal energy for browning.

Also associated with the prior art power conversion units in prior art ovens are rather elaborate schemes for detecting the output line voltage level and feeding a portion of that detected level back through an elaborate power supply so that power supplied to the magnetron tube is constant. In accordance with the principles of this invention, this elaborate circuitry and extra equipment is avoided by utilizing a simple printed power control circuit which is sensitive to microwave power and which is an integral part of the microwave unit. As the microwave unit varies its output power, a power control circuit of this invention varies the coupling between the electron beam and an interaction structure so as to assure a constant power and constant performance for the microwave unit without regard to either short or long time variations in the voltage supply, and without regard to aging effects manifested by decreased electron emission. In one embodiment the control circuit comprises a differential deflection amplifier which employs one distinct portion of the main electron beam for its amplifier beam. A printed microwave power sensitive device forms part of a printed bridge circuit which applies voltages to deflection electrodes positioned on opposite sides of the main electron beam. Such deflection voltages are of appropriate polarity and strength so as to automatically move the beam away from or toward the interaction circuitry so that a constant microwave power output is assured.

The foregoing and other features and objects of this invention may more readily be appreciated by the detailed specification, claims and drawings hereinafter in which:

FIG. 1 is a combined perspective and cutaway view of and integral microwave radiating and generating unit in accordance with this invention;

FIG. 2 is front perspective view of an oven constructed in accordance with the present invention where substantially the upper oven wall is formed by the microwave unit of this invention, said unit employing a plurality of microwave radiating means;

FIG. 3 is a circuit schematic of a voltage multiplier circuit;

FIGS. 4A and 4B are plan views of printed electrostatic focusing arrays in accordance with this invention;

FIGS. 5A through 5C are plan views of printed interaction circuitry of this invention;

FIG. 6 is a side elevation of an alternative embodiment wherein dielectric sheets form a vacuum envelope;

FIG. 7 is a combined perspecive and sectional view of an alternative embodiment of the microwave unit of this invention employing collector plates which serve the dual function of beam collection and thermal radiation; and an electron gun, both of which are offset from the main axis of the electron beam;

FIG. 8 is a combined perspective and sectional view of a further alternative embodiment of this invention;

FIG. 9 is a side elevation of a microwave unit employing two electron beams sharing a common microwave interaction structure;

FIG. 10 is a circuit schematic of a power control circuit in accordance with the principles of this invention;

FIG. 10A is a chart of output power as a function of microwave oven operating time useful in promoting a full understanding of the operation of the power control circuit of this invention; and

FIG. 11A and FIG. 11B are alternative embodiments wherein the fiat electron beam is rolled into a hollow cylindrical form.

Turning now to FIG. 1, a microwave oven unit in accordance with this invention comprises a compact and integral power conversion and ultrahigh frequency radiating and generating unit 15. This unit 15 includes a pair of opposed plane dielectric sheets 10 and 11. These sheets may be any suitable dielectric such as ceramic, glass, Pyroceram or other similar dielectric materials capable of receiving and supporting printed circuitry. Assuming that ceramic is used, for example, the bottom ceramic sheet 11 is selected shorter than the top ceramic sheet 10 by an amount sufficient to expose the electron beam collecting plates 21 and 22.

Ceramic sheets 10 and 11 are supported with proper spacing for the electron beam by a plurality of metal or ceramic posts. One such post 6 is shown broken away in FIG. 1. Post 6 includes a hollow cylinder having the microwave output lead 73 printed on the inner surface for connection to a microwave radiating antenna 81. Electrical interconnections, as necessary, may be provided in a simi* lar manner on the same or other posts not shown.

Opening 12 is the beam forming area and houses an electron gun 14, a heater 16, power conversion printed circuitry 17, and a plurality of printed beam converging electrodes 18. Communicating with the rectangular opening 12 is a narrower elogated fiat opening 20 which also runs across the width of ceramic sheets 10 and 11 as well as along the length thereof. This flat opening 20 is a beam guiding area and includes printed focusing circuitry 23, 24 on its upper and lower areas, respectively, for guiding the flat sheet electron 'beam 30 from gun 14 to opening 26. Opening 26 is a beam collection area which houses the electron beam collecting plates 21 and 22. These beam collection plates 21 and 22 are formed from narrow metal strips having areas selected such that the strips assume a high temperature for effective heat radiation through the glass envelope 25 which encases the entire oven unit. These thermal radiating and beam collection plates 21, 22, also may serve as diode cathodes as will be described in detail hereinafter.

Glass enclosure 25 encases the ceramic sheets 10 and 11 and is evacuated and vacuum sealed in any wellknown manner. Thus, openings 12, 20* and 26 between the ceramic sheets 10 and 11 are maintained at a high vacuum, required for electron emission and focusing and collecting the flat sheet electron beam 30.

The power conversion circuitry 17 of this invention may more readily be understood by reference to the voltage multiplier circuit 17 of FIG. 3. The voltage multiplier circuit 17 of FIG. 3 converts A.C. line voltage applied at terminals 45 to a ripple or DC. voltage of any desired maximum D.C. level in accordance with the number of diodes employed. For example, when three diode pairs are utilized, as shown in FIG. 3, a maximum voltage of 1,000 D.C. volts may be obtained from an A.C. input of 220 volts. If the unit is operated on an A.C. input line voltage of volts, then six diode pairs are required to generate a maximum DC output of 1,000 volts. Several different valued DC. voltage levels are developed by each pair of diodes 31 through 36. For example, distinct voltage levels are depicted at outputs 60 through 63 as voltage level V through V These D.C. voltage levels are utilized in the unit of this invention to form the electron beam. They are also used in various voltage combinations to focus the electron beam adjacent to a microwave interaction structure, and are also employed in various voltage combinations to collect the electron beam once it has passed adjacent to the microwave interaction structure.

In the voltage multiplier circuit 17 each diode pair has an anode of one connected to the cathode of the other diode in the pair. These pairs are further connected in series as shown. The voltage multiplier 17 of FIG. 3 connected in this manner operates in a conventional manner and includes capacitors 41 through 43, and capacitors 51 through 53.

All of the input capacitors 41 through 43 have one plate connected to a common terminal of the pair of input terminals 45. 'Input line voltage is applied through capacitors 41 through 43 to a common junction between an anode and cathode of the doubler diodes pairs 31, 32, and 33, 34 and 35, 36, respectively. Each of the doubler circuits further includes output capacitors 51, 52 and 53 connected across the doubler diode pair 31, 32, and 33, 34 and 35, 36.

It should be understood that the voltage multiplier unit 17 could be located externally of themicrowave unit 15 and output leads 60 through 61 would extend through the glass envelope 25 for connection to appropriate components in unit 15. The external diodes may be any conventional diodes such as thermionic or semiconductor to cite some typical examples. Inasmuch as the opening 12 is at vacuum pressure it may be desirable to fabricate the diodes 31 through 36 internally, and print or otherwise fabricate the capacitors, which take more room, externally. Of course, the entire multiplier circuit 17 may be located internally as another alternative. Each of these alternatives are discussed hereinafter in connection with the description of the operating principles of this invention.

The voltage multiplier circuit of FIG. 3 is depicted with internal diodes in FIG. 1, wherein the anodes of each of the diodes 31 through 36 are printed at selected locations along the width of ceramic sheets v and 11 within vacuum enclosure 12. These anodes are identified collectively as printed circuits 37. The cathodes of the voltage multiplier diodes of power conversion circuit 17 are formed from flat metal strips 38. These strips 38 may be stamped nickel strips which are brazed to ceramics 10 and 11 at the conical foot sections 39 and are sprayed with an electron-emitting coating. In FIG. 1, no attempt is made to precisely illustrate the printed circuit counterpart for the power conversion schematic of FIG. 3. However, printed circuit techniques for reducing conventional circuit schematics to corresponding printed circuits of the exact electrical configuration are well known. In such printed circuitry of FIG. 1, output power, as desired, is achieved by a predetermined electrode area for both the anodes and cathodes of diodes 3 1 through 36. This electrode area is essentially constant for any required output power whether 110 volt AC. or 220 volt A.C. line voltage is employed. Accordingly, once the printed circuit design has been completed by techniques well known in the art, the number of diodes required has little effect on the overall cost of the unit. This generalization is also true for capacitors 41 through 43 and 51 through 53, FIG. 3, when such capacitors are formed of alternate plated layers of conductive and dielectric materials as described in detail hereinafter.

Selected ones of the voltages V through V supplied by the power conversion circuitry 17 of FIG. 3, as described hereinafter, are connected to the heater coil 16 and electron gun 14; to the beam converging electrodes 18; to the focusing electrodes 23, 24; and to the beam collector plates 21 and 22. Electron gun 14- may be any well-known electron gun, provided that it is employed to form a fiat sheet electron beam. For example, the electron gun 14 of this invention may be fabricated similar to the cathodes 38 of the diodes in the power conversion circuitry 17, in that a stamped, nickel strip is sprayed with an emissive coating so that electrons are emitted from the concave surface when heater coil 16 is energized. Heater coil 16 comprises a high resistance wire such as tungsten wire which is connected to voltage circuit 17 or to the AC. line voltage. This tungsten wire is wound around a ceramic rod supported in opening .12. This voltage, in a wellknown manner, heats coil 16 which in turn heats the cathodes 38 of the doubler diodes and heats electron gun 14. A steady stream of electrons are emitted from the emissive coating of heated gun 14. The electrons emitted from the curved surface of an electron gun 14 are convergd into a fiat sheet electron beam 30 by converging electrodes 18'. The high current density of the electron beam would cause the beam to spread in thickness in area 20 due to mutual repulsion of the electrons forming the beam 30. Beam 30, however, is held essentially in the flat sheet beam as shown by the dashed lines of FIG. 1 by a pair of electrostatic focusing arrays 23, 24, one each of which is located above and below beam 30.

It should be understood that numerous focusing arrays are available. For example, the focusing array in FIG. 1 is repeated in FIG. 4A as one possibility. The focusing array of FIG. 4A includes two interleaved ladder circuits 23 and 24. Each one of the circuits 23 and 24 are electrically connected to different voltages of the voltage multiplier circuit 17. Electron beam 30 has an average electron beam voltage. The voltages for circuits 23 and 2 4 are selected to be both above and below this average voltage for beam 30'. These voltages at circuits 23 and 24 establish field gradients in opening 20 which alternatively attract and repulse beam 30 toward and away from the focusing arrays on both ceramic sheets 10 and 11. The net result of this field gradient is an inwardly directed electric field that keeps the electron beam 30* focused substantially as a fiat sheet beam. For increased focusing and higher current density, a multiple voltage focusing array as shown in FIG. 4B may be employed. Typical voltages for this multiple voltage array of FIG. 4B may be the voltages V V on the lower tortuous array leads 54, 55 and voltages V and V on the upper tortuous leads 64 and 65.

As is well known, the electron beam 30 represents a large amount of kinetic energy. This kinetic energy is present because the electrons are accelerated from the electron gun 14 to the beam collecting area 26. The kinetic energy of electron beam 30 is converted into microwave energy within unit 15. To achieve this conversion, a microwave interaction structure 71 is printed on the upper surface of the ceramic sheet 10. Numerous forms of microwave interaction structures are available. The form shown in FIG. 1 is an array of printed parallel wires 72 connected at their centers by a printed lead 74 so as to form a flattened ring bar helix. This ring bar helix, of selected dimensions, is so positioned that a microwave signal is propagated through the ring bar helix at the same axial velocity as the electron beam. Microwave fields are established by the microwave signal passing through the ring bar helix 71, and such fields extend through the ceramic sheet 10 and interact with the electnon beam 30. This interaction between the signal in helix 72 and beam 30 results in the generation of microwave power.

Generating microwave power from an electron beam is a typical conversion operation for all beam type microwave tubes. S uch tubes may be of the traveling wave t ube type or of the klystron type. Both of these types include microwave interaction structures and result in considerable amplification. The difference in operation of the two types depends upon the type of interaction struct-ure employed. Several forms of interaction structure for both types of operation are depicted in FIGS. 5A through 5C. In each of the interaction structures depicted in FIGS. 5A through 5C, an array of parallel wires are printed on the ceramic sheet and have the required property of propagating a microwave signal at the same velocity as the electron beam. In FIG. 5A, the interaction structure is formed by two separate parallel arrays 46 and 47 Which are interleaved to form an interdigital line. In FIG. SE, a single printed wire 48 is formed in a tortuous path of interconnected parallel lines as a meander line.

FIG. 5C depicts an interaction structure that is available to obtain a klystron type of operation. In a klystron, the electrons act only at intervals along the interaction structure of 5C. These intervals of interaction are actually sections of a ring bar helix identified as 56, 57 and 58 in FIG. 5C. These sections are terminated by shorting bars 50. There is no interaction achieved by shorts 50 between each of the ring bar helix sections 56 through 58 and in these shorted sections the electron beam merely drifts without any interaction between it and the signal being propagated through the interaction structure of FIG. 50.

Comparison between the interaction structure of FIG. 5A and the foregoing described focusing array 23, 24 of FIG. 4A reveals that the two arrays are very similar. With respect to the focusing array 23, 24 of FIG. 4A which is itself an interdigital line, no interaction between a signal in the focusing array and the electron beam takes place because the velocity of propagation in the focusing array is chosen to be much less than the velocity of the electron beam. On the other hand, the velocity of propagation of the signal in the interdigital line of FIG. A is chosen to be equal with the velocity of the electron beam as this is an essential requirement for operating either in a traveling wave mode or a klystron interaction mode. By separating the focusing array 23', 24 and the microwave interaction structure and placing them on opposite sides of the ceramic sheet 10, FIG. 1, each of these circuits may be optimized independently of the other, whereby a higher conversion efficiency and higher gain is obtained. However, these structures could be combined into a single structure if so desired.

With reference to FIG. 1, however, it should be understood that the microwave interaction is not of itself selfoscillatory. If self-oscillation is not established, then a source of input microwave power for the microwave interaction structure is required. The necessity for any external microwave power source can be eliminated by employing the printed feedback circuitry 75 of FIG. 1. This feedback circuitry includes a printed directional filter 75 shown in dashed lines in FIG. 1. This directional filter inductively samples a small portion of the output microwave power at the output lead 73 of the ring bar helix.

This inductive coupling is achieved by the printed J- shaped stub 74 which has induced therein a sampled signal represented by arrows 76. This sample of the output signal is passed through the printed directional filter squares 77 and 78 and coupled back through another I- shaped printed stub 79 and lead 80 to the input of the ring bar helix 72. The printed directional filter squares 77 and 78 are dimensionally selected to provide a pass filter having a narrow pass frequency range. Accordingly, the entire microwave oven unit oscillates only at the pass frequency of the directional filter 75. For example, one typical pass frequency particularly appropriate for a microwave oven is an oscillating frequency of 915 or 2,450 megacycles.

In FIG. 2 a microwave cavity or oven enclosure 90 is shown having four different random antennas 81 through 84, forming substantially the upper wall '91 of the oven cavity 90. As is well known in the art, several different microwave frequency modes exist in a typical oven enclosure 90. These different modes depend upon the enclosure dimensions which generally speaking are several wave lengths long. The dimensions of the printed circuit antenna are selected so the right amount of microwave power is coupled to all modes whereby uniform power and heating density is provided in the oven enclosure. As depicted in FIG. 2, the microwave oven unit may comprise one wide flat beam having several interaction structures each having associated therewith antennas 81 through 84. Each interaction structure may operate at a frequency different from all other structures. Of course, a plurality of separate microwave power units, each of them described hereinbefore in connection with FIG. 1, may be employed rather than having one wide beam. These different units may be separately encased or encased in a common vacuum envelope 100'.

Employing a plurality of antennas and several different frequencies for each, in accordance with this invention, provides several advantages even though such an arrangement does not yield coherent microwave power. Numerous prior art designs for traveling wave tubes have strived to achieve coherency. 'However, I have found a distinct advantage in noncoherency. By placing as many microwave antenna circuits, such as 81 through 84, FIG. 2, side by side as is desired, coherency is destroyed, but the energy coupling within the oven cavity 90 is optimized for a wide variety of foods. Furthermore, by utilizing antennas printed in random patterns, microwave energy is radiated from several different points in the oven, and even more uniform energy distribution is thereby achieved. In FIG. 1 the antenna 81 is printed on the lower surface of dielectric sheet 11. The thickness of the dielectric sheet is properly chosen so that the focusing array 23 and 24 forms the necessary ground plane for the antenna. The antenna 81 radiates the microwave energy through the glass envelope 25. It should be realized that the antennas 81 through 84 may be printed on the lower surface of the glass envelope 25 and connected by conventional techniques to the interaction output lead 73, FIG. 1. Also, it should be appreciated that the antennas may, if desired, be designed completely separate from the microwave unit, and conections may be made in any conventional manner [from the output of the interaction stnucture to the separate antennas.

The microwave antenna structures 81 through 84 establish a high frequency radiation for accomplishing what is commonly referred to as a deep cooking effect on food placed within the oven cavity 90. In addition, the microwave unit 15 of FIG. 1 is capable of supplying thermal energy in the form of high temperature radiant heat created by beam collection; or such heat may be dissipated by conduction and convection from the ceramic and collection plates in the beam collection area of the microwave unit 1'5.

As a general rule the generation of microwave power utilizes approximately one-half of the original kinetic energy of electron beam 30. The interaction operation tends to establish classes of electrons with each class having a different velocity. The several voltages V through V from the voltage multiplier circuit are each individually connected to independent collection plates 21 and 22 to maximize efiiciency. Classes of electrons with slow velocity are collected by the higher voltage collection plates and higher velocity electrons are connected by lower voltage collection plates.

The beam collector is described in more detail with respect to FIG. 1 in which radiated heat for browning is supplied by collector electrodes 21 and 22. Associated with electrodes 21 and 22 are a plurality of electron beam deflection plates 85 through 87. These plates 85 through 87 are printed on the ceramic sheets 10 and 11 near the junction of the elongated beam guiding opening 20 and the electron beam collecting and the heat radiating area 26, and are connected to appropriate voltages from the voltage multiplier circuit 17 so as to deflect the electron beam 30 upward toward the thermal radiation collecting plates 21 and 22. These radiation plates are selected with areas appropriately designed so that the narrow metal strips operate at approximately 1,000 degrees centigrade. This high temperature allows effective heat radiation through the envelope 25 which heat may be used for browning. In these instances where a controlled degree of browning is desirable, any conventional control 89, FIG. 2, may be utilized to vary the amount of voltages applied to the electron beam deflection plates, whereby the temperature of the plates is regulated.

Furthermore, in accordance with the principles of this invention, the collector plates 21 and 22 in addition to beam collection and thermal radiation, may also serve as cathodes for diodes forming part of the voltage multiplier circuit 17. Thus, electrodes 21 and 22 may be coated with any suitable electron emissive material so that the collector plates 21 and 22 operate as cathodes for selected parts of the diodes required for the voltage multiplier circuit 17, FIG. 3.

In the description hereinbefore of the voltage multiplier diodes located in the electron gun enclosure 12, heat for the cathodes 38 was supplied by the cathode heater unit 16. In this alternate embodiment, only part of each diode is located in the electron gun enclosure 12 and heated by the heater 16. These parts permit the formation of a low-current beam which heats the diodes located in the beam collecting and radiating area 26. As these collector diodes 21 and 22 increase in temperature, the doubler circuit provides the required voltage for obtaining full electron beam current. Anodes 28 and 29 are again printed opposite the heated cathodes 21 and 22, and are connected to appropriate capacitors by printed circuit leads described in more detail hereinbefore.

A different form of combination beam collecting and heat radiating surfaces is shown in FIG. 7. The electron beam collection plates 92, 93 and 94 of FIG. 7 are curved metal strips mounted across the width of the oven unit 15, on a suitable mounting structure 125. Convex surfaces of plates 92A through 94A are provided for heat radiation. These convex surfaces are positioned on mounting 125 so as to establish overlapping thermal radiation patterns. By staggering these thermal radiation areas in the front and back sides of the top of cavity 90, FIG. 2, uniform browning is thus achieved without requiring any additional oven coil. Microwave radiation is achieved by antenna 81 printed on a third dielectric sheet 9, in this alternate embodiment. The spacing between 9 and 11 is selected to provide the necessary ground plane for antenna 8.1.

Electron gun 14, FIG. 1, is positioned along the velocity axis for the electron beam 30. In the formation of the electron beam 30, small particles of the emission surface as well as electrons may be emitted from the electron emitting surface of electron gun 1-4. Such small particles of the emission material may become lodged between the alternate electrodes of the electron beam cOnVerging and focusing arrays. Such deposits can short out some of the beam guiding electrodes and thus reduce tube life and tube efficiency. This possible disadvantage is avoided by employing an electron gun of different configuration as shown, for example, by the electron gun 66 of FIG. 7. This electron gun 66 is fabricated and supported in substantially the same manner as electron gun 14 of FIG. 1, however, its electron emitting surface is offset from the axis of the electron beam 30. The electron beam 30 is converged as before and in addition bends into the electron beam axis of opening 20. Converging and bending as required to guide the beam into the flat elongated beam guide opening 20 is achieved by a plurality of formed metal electrodes 95, 96, 97 and 98. These electrodes are brazed or otherwise bonded to the ceramic sheets 9 and 10, and receive voltages from appropriate outputs of the printed voltage multiplier circuit 17, FIG. 3. Bending the electron beam 30 in the manner depicted in FIG. 7 assures that all emitted particles of the emission surface are propelled into the underside of ceramic sheet and are not allowed into the beam guiding area 20. The possibility of electrode shorting is thus virtually eliminated by positioning the electron emitting area at an angle either above or below the main velocity axis of the electron beam.

As mentioned in the introduction to this specification, prior art microwave units generate an output power which decreases with the life of the tube unless elaborate power supply units are employed to compensate for this variation. In FIG. 10A a chart of output power with respect to tube operating time is shown having curve 130 based on an unregulated 220 AC. input voltage unit. It is clear from power curve 130 that the output power from a unit may start approximately at 2,000 watts but tends to decrease with operating time. Such output power reduction reflects a corresponding variation in the cooking time required for different foodstuffs.

A regulated power control curve 115 is also shown in FIG. 10A. Curve 115 represents an output power from the microwave unit of this invention at a constant 1,000 watt level throughout the entire life for the unit. Such constant output power assures a constant cooking time for any given food to be cooked in spite of the age of the microwave unit.

Constant output power of curve 115 is achieved by a new and novel power control circuit 110, FIG. 10, of this invention. Control circuit 110 measures the microwave power output from the tube in accordance with the heating effect created by such microwave power, and converts this measurement in a simple and straightforward manner to a deflection voltage which automatically varies the degree of interaction so as to compensate for any heat, i.e., power loss. For a new microwave tube 15, the power control circuit initially positions electron beam 30 near the bottom focusing array as shown in FIG. 7. As the tube ages, the electron beam 30 moves upward toward the interaction structure 71 so as to interact in a more eflicient manner, and thus generate more microwave power output. Although described thus far with respect to long-term performance degradation, shortterm variations or fluctuations in input voltage are also compensated for by control circuit of this invention as will become clear in the detailed description hereinafter.

iPower control circuit 110, as is true of most of the operative structures for the microwave unit 15, is advantageously a printed circuit. For simplicity of description, however, the power control circuit is shown in combined pictorial and circuit schematic form in FIG. 10. The power control circuit 110 includes beam deflection plates 104 and 105 which are shown in FIG. 7 positioned across the width of the flat electron beam on opposite sides of electron beam 30'. These deflection plates 104 and 105 move the main electron beam 30 in a direction 106 which is transverse to its main electron velocity axis 107. It should be understood that when the main electron beam 30 is deflected upon entry into the beam guiding area 20, the focusing electrodes 23, 24 of FIG. 7 maintain beam 30 in the initial position throughout beam guiding area 20. Thus, if the beam 30 is initially deflected by deflection voltages at plates 104 and 105, downward toward the bottom focusing array, the beam holds this position as it passes through beam guiding area 20 until the deflection voltage across plates 104 and 105 changes the initial position of beam 30.

FIG. 10, in addition to the deflection plates 104, 105 which control the position of the electron beam relative to the microwave structure 71, comprises a resistance Wheatstone bridge 140, having in one arm thereof a means 141 for sensing microwave output power. An amplifier 150 is connected between the output terminals 143 and 145 of the bridge 140 and the deflection plates 104 and 105. Means 141 senses any variation in heating power from unit 15 and may advantageously be any known electrical component in which the resistance varies proportionately to changes in temperature in the component 141. For example, means 141 may be a bolometer which measures microwave power by is heating effect. As shown in FIG. 7, bolometer 141 may advantageously be positioned at a section of the output lead 73 from the ring bar helix 72 and may take the form of printed variable resistor or a thin tungsten wire. Microwave heating effects in the tungsten wire change the wires resistance and this resistance change in turn is converted to a voltage change by the resistance bridge 140. In resistance bridge 140, each resistor 146 is a heat compensated fixed value resistor. The bolometer 141 of FIG. 7 has a resistance which varies as microwave power varies, as shown schematically in dashed form in the remaining arm of bridge circuit 140.

In the representative curve shown in FIG. 10A, it was assumed that a constant output power for the microwave unit would be 1,000 watts. This output power value establishes a predetermined initial resistance value in bolometer 141 which value is chosen with respect to the resistance values of fixed resistors 146 so as to initially balance bridge circuit 140. Input voltages V and V are applied 'across circuit 140* and with bridge circuit 140 balanced, there is no output voltage signal present on the bridge output leads 143 and 145.

Input voltages V and V also serve as electron beam forming voltages for both the main electron beam 30 and a secondary electron beam 130. This secondary electron beam may be an outer edge of the main beam 30 or it may be an entirely separate electron beam. Associated with both electron beams 30 and 130* are beam converging and focusing electrodes 95 through 98. These converging electrodes were discussed in detail with respect to the operation of unit 15, FIG. 7, and need not be repeated here. Their function is to converge and form the electrons emitted as a result of the voltage difference between the electron gun 66 and the anodes 97 and 9 8, into flat sheet electron beams 30 and 130.

The secondary electron beam 130 allows a difference amplifier 150 to be readily and economically incorporated as an integral operative structure in the microwave unit 15. It should be understood that any suitable amplifier means could be employed in place of differential deflec tion amplifier 150. For supplying output power correction voltages to deflection plates 104 and 105 this amplifier 150, however, is particularly applicable to the feedback servo operation of this invention because small variations in output voltage signals from bridge circuit 140 are amplified to large magnitude differential correction voltages which are independent of errors normally associated with many amplifier means, particularly if two amplifiers were to be employed. I

In the operation of differential deflection amplifier 150, when bridge circuit 140 is balanced and there are no output voltage signals present on leads 143 and 145, the electron beam 130 divides equally between the upper and lower collection plates 151 and 152. A resistor 155 is connected between upper deflection plate 151 and the input voltage V Input voltage V is also applied to the main electron beam deflection plate 105. Resistor 155 is chosen to have a resistance value which represents a predeterrmined voltage drop with bridge 140 balanced. Output lead 156 from amplifier 150 connects plate 151 directly to the deflection plate 104. This connection establishes an initial voltage gradient across plates 104 and 105, which acts to deflect the electron beam 130 downward in the position shown in FIG. 7.

In order to fully appreciate the operation of control circuit 110', assume that the power output as represented by the resistance value of bolometer 141 decreases either due to age or because of a variation in the input voltage for the microwave oven unit. This decreased output power represents a lower temperature in the bolometer 141 lWhlCh in'turn is manifested by a variation in the resistance value of bolometer 141. Bridge 140 is consequently unbalanced and an output voltage signal on leads 1-43 and 145 applies a voltage gradient across the deflection electrodes 147 and 148 of the difference amplifier 150. This voltage disturbs the normal division of the electron beam 130 to plates 151 and 152, and thus the current through resistor 155 develops a difierent voltage gradient that is applied between the deflection plates 104 and 105. This new voltage gradient deflects the entire electron beam 30 upward so as to increase the degree of interaction between beam 30 and the microwave interaction structure 71, FIG. 7. Increased interaction, of course, increases the power output from the microwave unit 15, which, in turn, is reflected in bolometer 141 so as to rebalance bridge circuit 140*.

Power control circuit 110 also compensates for periodic increases in microwave output power by reducing the degree of interaction between electron beam 30 and the microwave interaction structure 71, FIG. 7. This operation need not be described in detail because it is evident from the previous description. In short, therefore, the power control unit 110 of this invention increases or decreases the amount of interaction between microwave structure 71 and the electron beam 30 in a manner which assures constant output power throughout the entire tube life.

FIG. 6 depicts a side elevation Olf an alternative embodiment of a microwave oven unit in accordance with the principles of this invention. In IFIG. 6, two opposed plane dielectric sheets 10 and 11 are bonded together in an air-tight seal at the butt joints for the dielectric sheets. Numerous known techniques are available for bonding ceramic sheets together and for forming an interior vacuum. After the interior cavities are rough vacuum pumped, the structure is sealed off, and final vacuum pumping may be accomplished by activating getter 7. An electron beam 30 is formed, guided and collected by printed circuitry described hereinbefore.

In the microwave unit 15 of FIG. 6, the interaction structure 71 is located on top of sheet 10 along with associated printed feedback circuitry described earlier. Connection to an antenna 81, which is located on the bottom of ceramic sheet 11, is made by way of a printed microwave lead 73 which wraps around the beam collection end of ceramic sheets 10 and 11.

A plurality of ofiiset beam collection plate pairs 111, 112 and 113 are printed on opposite upper and lower surfaces of the beam collection area 26. These plates are connected by printed circuit connections to voltage multiplier output leads 60 through 63, FIG. 3, and thus are at the voltages shown in FIG. 6. As electron beam 30 interacts with microwave structure 71 classes of different velocity electrons are established in beam 30. Slower velocity electrons are deflected earlier than faster electrons. Collection plates 111, 112 and 113 are selected to have different areas so that these different classes of electrons are collected by plate areas which provide proper heat transfer. Heat formed by such electron beam collection is dissipated by conduction through ceramic sheet 11, by radiation and natural convection *from unit 15.

FIG. 11 depicts cross sectional perspectives of two alternative embodiments of the microwave oven unit of this invention. 'In FIG. ll-A, the electron beam is a flat beam that is rolled into a hollow cylinder. The electron beam 180 traverses the vacuum opening in a circular path from the gun 161 mounted on the inner ceramic cylinder 162. The electron beam is collected on a plurality of collecting plates 165 through 167 which are located on the innermost surface of outer cylindrical cenamic 170 so as to conduct heat from unit 15 by conduction or normal convection. Collecting plates 165 through 167 may, as an alternative be mounted so as to radiate thermal heat through the glass envelope 171 which enc'ases the entire unit. Structures for focusing the electron beam 180, and microwave interaction and microwave radiation antenna structures are not depicted in FIG. 11A, however, these structures operate in the same manner as those described hereinbefore in accordance with the principles of this invention. The hollow cylindrical unit 15 of FIGS. 11A and 11B offers an additional advantage in that most glass envelopes for tubes are readily available in a cylindrical design and thus this structure is more conventional and the glass vacuum envelopes may be fabricated with less difficulty than fiat glass envelopes of FIGS. 1 and 7.

In FIG. 11B, another form of hollow electron beam is depicted in which the electron gun 181 is in the form of a washer. In this configuration of FIG. 11B the electron beam 184 is a hollow beam which moves axially along the length of cylinder 185.

FIG. 8 depicts a still further alternative embodiment of this invention wherein a thick ceramic base is joined with an air-tight seal to a metal cover 191. In the microwave unit of FIG. 8 both the electron gun and the collector electrode are displaced above the main velocity axis of the electron beam 195. In this microwave unit only flat ceramic sheets are employed, thus providing simplification in the tfiabricating process for the microwave unit 15.

In the unit of FIG. 8, the capacitors 141 through 143 and 151 through 153 for the voltage multiplier 17 of FIG. 3 are shown symbolically as a single capacitor unit 196 positioned on top of the metal cover 191. Capacitor unit 196 is comprised of alternate layers 192 of dielectric, and layers 193 of current conducting material. The dielectric, for example, may the any suitable material having a high dielectric constant such as barium zirconate. The material for conductive layers 193 may be printed layers of any suitable electrically conductive material. Numerous other dielectric and conductive materials, known in the art, may be utilized for the printed capacitor unit 196. In such capacitor units it is common practice in the art to join areas of selected layers of dielectric and conductive material with a printed or common output lead so that several distinct capacitors may be formed from one large capacitance plate. These individual capacitors are connected to the diodes located in the vacuum area 12 in any, suitable manner, so as to form a voltage multiplier circuit 17 discussed in detail hereinbefore.

In FIG. 9, two separate electron beams 176 and 177 interact with a common printed interaction structure 186 which is sandwiched between two cenamic sheets 187 and' '188. This common interaction structure 186 and individual interaction structures 18?: and 184 provide a more effective power output 'level than a single beam and single interaction structure.

It is to be understood that the foregoing features and principles of this invention are merely descriptive, and that inany departures and variations thereof are possible by those skilled in the art, without departing from the spirit and scope of this invention.

What is claimed is:

1.: Au ultrahigh frequency energy generator comprismg:

(a) means forming a vacuum enclosure;

(b) a source of standard line voltage connected to said vacuum enclosure means for supplying line voltage within said vacuum enclosure;

(c) a voltage multiplier circuit located in said vacuum enclosure and connected to said line voltage supplying means for establishing a plurality of successive higher valued voltages;

(d) means at one end of said vacuum enclosure for emitting a stream of electrons and focusing means associated with said emitting means and connected to voltages of said voltage multiplier circuit for accelerating and focusing said emitted electrons in the form of a flat sheet electron beam;

' (t electron collecting means positioned at the opposite end of said enclosure and connected to a plurality of said successive voltages of said voltage multiplier circuit for collecting the emitted stream of electrons;

(f) a first and a second flat dielectric sheet spaced on opposite sides of said electron stream;

(g) a first pair of distinct tortuous printed electrical circuits interleaved on the surface of said first dielectric sheet and facing said electron stream;

(h) a second pair of distinct tortuous printed electrical circuits interleaved on the surface of said second dielectric member facing said electron stream;

(i) printed circuit means connecting distinct ones of said first and second pair of tortuous electrical circuits to one of said voltages of said voltage multiplier circuit, and for connecting the remaining one of said first and second pair of tortuous circuits to a secondvoltage of said multiplier circuit, said first and second voltages selected to focusing said emitted electrons into a flat electron beam between said first and second dielectric sheets; and

(j) a convoluted electrostatic structure printed in a planar configuration on the remaining surface of said second dielectric sheet for interacting with the moving electrons in said beam for propagating a microwave signal therethrough at the same velocity as of the said electron beam.

2. An ultrahigh frequency generator comprising:

a pair of opposed planar dielectric sheets defining therebetween a substantially fiat elongated vacuum cavity;

means at one end of said vacuum cavity for emitting an electron beam, and means at the other end of said cavity for collecting said electron beam;

electrostatic focusing means printed on the planar surfaces of said opposed dielectric sheets which define said cavity for focusing the electron beam into a flat sheet beam positioned between the dielectric sheets; and

a microvae interaction structure printed on at least one of the dielectric sheets and interacting with the moving electrons in said beam for converting electron translational energy in said beam to microwave energy in said printed microwave interaction structure.

3. An ultrahigh frequency generator in accordance with claim 2 wherein:

said vacuum cavity is defined by said dielectric sheets being sealably joined at a raised periphery thereof.

4. An ultrahigh frequency generator in accordance with claim 2 wherein:

said vacuum cavity is formed within a sealed glass envelope encasing said dielectric sheets.

5. An ultrahigh frequency generator in accordance with claim 2 wherein:

said microwave interaction structure comprises at least two separate configurations distinct from each other and each adapted for converting said electron translational energy in said beam to microwave energy having diiferent microwave frequencies for each separate configuration.

6. An ultrahigh frequency generator in accordance with claim 2 and further comprising: l

antenna means printed on at least one of said dielectric sheets; and

means connecting said antenna means to said interaction structure for radiating said microwave energy away from said generator.

7. An ultrahigh frequency generator in accordance with claim 6 wherein:

said connecting means is a printed electric conductor.

8. An ultrahigh frequency generator in accordance with claim 6 wherein:

said antenna means is tion.

9. An ultrahigh frequency generator in accordance with claim 2 wherein said electrostatic focusing means comprises:

a pair of printed circuits with one each of said pair printed on one each of said dielectric sheet surfaces in face-to-face relationship.

10. An ultrahigh frequency generator in accordance with claim 9 wherein:

said interaction structure is printed on a dielectric sheet surface common to one of said printed electrostatic focusing means.

11. An ultrahigh frequency generator in accordance with claim 9 wherein:

said interaction structure is printed on one dielectric sheet surface opposite from that sheets surface which has said electrostatic focusing means printed thereon.

12. An ultrahigh frequency generator in accordance with claim 10 and further comprising:

antenna means'printed on one of the remaining planar surfaces of said pair of dielectric sheets opposite to the surface having said electrostatic focusing means printed thereon; and printed conductor means connecting said antenna means to said interaction structure. 13. An ultrahigh frequency generator in accordance with claim 2 and further comprising:

feedback means coupled to said printed conductor and to said interaction structure for causing said unit to oscillate at a predetermined frequency. 14. An ultrahigh frequency generator in accordance with claim 13 wherein:

said feedback means comprises a filter printed on one of said dielectric sheets for passing said predetermined frequency. 15. An ultrahigh frequency generator in accordance with claim 2 wherein:

a randomly zig-zag configurasaid means for collecting said electron beam comprises at least one collection plate printed on one of said dielectric sheets at the end of said cavity.

16. An ultrahigh frequency generator in accordance with claim 15 wherein:

said printed collection plate further comprises:

a plurality of individual plates;

a plurality of successively increasing valued voltages; and

means applying one each of said successively increasing valued voltages individually to one each of said plurality of beam collection plates Whereby electrons moving in said sheet at different translational velocities are collected by different ones of said collection plates.

17. An ultrahigh frequency generator comprising:

a pair of opposed planar ceramic sheets defining therebetween a substantially flat elongated vacuum cavi- W;

means at one end of said vacuum cavity for emitting an electron beam, and means at the other end of said cavity for collecting said electron beams;

electrostatic focusing means printed on the planar surfaces of said opposed ceramic sheets which define said cavity for focusing the electron beam into a flat sheet beam positioned between the ceramic sheets;

a microwave interaction structure printed on at least one of the ceramic sheets and interacting with the moving electrons in said beam for converting electron translational energy in said beam to microwave energy in said printed microwave interaction structure; and

means for maintaining said microwave energy at a constant value during the operative life of said generator, said constant energy maintaining means comprising beam deflection means for controlling the degree of interaction between said electron beam and said microwave interaction structure.

18. An ultrahigh frequency generator in accordance with claim 17 wherein said beam deflection means further comprises:

means responsive to signals applied thereto for controlling said degree of interaction;

means for emitting signals proportional to microwave energy variations from said constant microwave energy value; and

signal applying means connected between said beam deflection signal responsive means and said signal emitting means.

19. An ultrahigh frequency generator in accordance with claim 18 wherein said signal responsive means of said beam deflection means comprises:

a pair of beam deflection plates, one each of said pair positioned on opposite sides of said flat sheet of moving electrons.

20. An ultrahigh frequency generator in accordance with claim 18 wherein said signal emitting means comprises:

microwave energy sensitive means coupled to said microwave interaction means for sampling the amount of electron translational energy converted to microwave energy; and

means for converting samples obtained by saidsampling mean to a first signal proportional to variations in said microwave energy above said constant value and to a second signal proportional to variations in said microwave energy below said constant value.

21. An ultrahigh frequency generator in accordance with claim 18 wherein said microwave energy sensitive means comprises:

a first resistance means having a predetermined fixed resistance selected for said constant value microwave energy; and

a second resistance means having value variations proportional to microwave energy variations from said constant microwave energy value.

22. An ultrahigh frequency generator in accordance with claim 18 wherein said signal applying means comprises:

a difference amplifier having input and output terminals;

means connecting said amplifier input terminal to said signal emitting means; and

means connecting said amplifier output terminal to said signal responsive means of said beam deflection means.

23. An ultrahigh frequency generator in accordance with claim 22 wherein said dilference amplifier comprises:

a deflection amplifier sharing one portion of said flat sheet of moving electrons as its operative electron beam;

means connected to said amplifier input for controllably deflecting said one electron beam portion in response to signals emitted from said signal emitting means;

means for collecting said One beam portion; and 7 means connected between said collecting means and said amplifier output for developing a first amplified signal proportional to variations in said microwave energy above said constant energy value, and for developing a second amplified signal proportional to variations in said microwave energy below said con stant energy.

References Cited UNITED STATES PATENTS 2,834,915 5/1958 Dench 3l5---39.3 2,866,917 12/1058 Salisbury 315-4- 3,058, O*25 10/1962 Hogg 315-393 X ROY LAKE, Primary Examiner. v SIEGFRIED H. GRIMM, Assistant Examiner.

US. Cl. X.R. 

